The advantageous mechanical properties, dimensional stability and optical properties of polyester films are well-known. However, polyester films are susceptible to hydrolytic degradation, which results in a reduction in the intrinsic viscosity of the polymer, and a consequent deterioration in one or more of the afore-mentioned desirable properties of the film, particularly the mechanical properties. Poor hydrolysis resistance is a particular problem when the film is used under humid conditions and/or elevated temperatures and/or in exterior applications, such as in photovoltaic (PV) cells.
In order to improve the hydrolysis resistance of polyester films, it is known to incorporate hydrolysis stabilisers into the film. For instance, the addition of carbodiimides as end-capping agents in polyester compositions was proposed in U.S. Pat. No. 5,885,709 and EP-0838500, amongst others, but such additives have a tendency to emit harmful gaseous by-products. US-2003/0219614-A1 reports that the use of polymeric carbodiimides as the hydrolysis stabilisers reduces the tendency for gas evolution. US-2002/0065346-A1 teaches hydrolysis stabilisers selected from a phenolic compound, an oxazoline and/or a monomeric or polymeric carbodiimide, optionally combined with an organic phosphite. GB-1048068 teaches the use of copper salts of organic carboxylic acids as hydrolysis stabilisers. U.S. Pat. No. 3,657,191 and U.S. Pat. No. 3,869,427 teach the modification of the terminal groups of the polyester by reaction with ethylene carbonates or monofunctional glycidyl ethers. Hydrolysis-resistant polyesters stabilised by the use of terminal epoxy group-containing compounds are also disclosed in EP-0292251-A. In EP-1209200 it is reported that a combination of a glycidyl ester and a glycidyl ether in the presence of a catalyst which promotes reaction between glycidyl and carboxyl groups improves the hydrolysis resistance of polyesters, although that disclosure is directed to polybutylene terephthalate (PBT), which crystallises much faster than PET, and its use in the manufacture of injection-moulded materials. U.S. Pat. No. 6,498,212 discloses polyesters in which hydrolytic stability has been improved by the use of a polymeric end-capping agent selected from epoxyethylene-ethyl acrylate copolymers, epoxystyrene-butadiene-styrene block copolymers and aminopolyethylene copolymers. The use of epoxidised fatty acid alkyl esters (such as epoxidised stearic acid 2-ethyl-hexyl ester) and/or epoxidised fatty acid glycerides (such as epoxidised soybean or linseed oil) as hydrolysis stabilisers in polyester compositions is disclosed in CA-2514589-A, U.S. Pat. No. 4,540,729, U.S. Pat. No. 5,589,126, U.S. Pat. No. 7,229,697, U.S. Pat. No. 7,241,507, US-2005/0137299-A1, US-2007/0238816-A1 and US-2007/0237972-A1. Other methods of improving hydrolytic stability of polyethylene terephthalate (PET) films include the simultaneous control of parameters such as intrinsic viscosity, diethylene glycol content and crystallinity, as disclosed in EP-0738749-A. The control of intrinsic viscosity and crystallinity, in combination with the presence of an antioxidant, is reported in EP-0620245-A as improving high-temperature (180° C.) ageing characteristics without detriment to in-plane delamination properties for polyester films used as insulator materials in electric motors and capacitors. U.S. Pat. No. 4,115,350 and U.S. Pat. No. 4,130,541 teach that the reaction product of polyesters with epoxidised alkyl esters of mono-carboxylic acids, amides and thio-acids improves the thermal stability of the polyester in fibres and cords made therefrom. U.S. Pat. No. 3,372,143 teaches that the reaction product of polyesters with epoxidised alkoxy- or aryloxy-ethers improves the dyeability of fibres made therefrom.
One of the problems associated with the incorporation of hydrolysis stabilisers into polyester films is that while increasing the concentration of the additive improves the hydrolysis resistance, it does so at the expense of a reduction in the melting point and a deterioration in the mechanical properties of the polyester film. One of the consequences of a reduction in mechanical properties is that the processability of the filmed polyester becomes poor, and breakage of the film web occurs during manufacture and subsequent processing.
Another problem with the use in the prior art polyester films of hydrolysis stabilisers based on epoxidised fatty acids, particularly epoxidised fatty acid glycerides, is that such additives have a tendency to decompose during film manufacturing and processing with evolution of acrolein, a highly toxic, flammable and foul-smelling substance.
An additional problem with the known hydrolysis stabilisers, particularly those based on certain epoxidised fatty acid glycerides and multi-functional glycidyl compounds, is the reduction of film quality and processability when such additives are incorporated into the film in an amount effective to provide improved hydrolysis resistance. In particular, such additives induce profile defects and unacceptable levels of die-lines in polyester films, i.e. poor uniformity in thickness and/or light transmission across the film web, and the extrudate can become impossible to process on a film-line because of breakage of the film web. It is believed that such problems are at least partly attributable to cross-linking and gel formation, which interferes with the stretching process experienced by the film during its manufacture. A further problem with using multi-functional glycidyl compounds as hydrolysis stabilisers for PET is that their higher rate of chain extension of the polyester increases melt viscosity, which in turn reduces the extrusion output at a given temperature, and this is economically undesirable. While viscosity could theoretically then be reduced by increasing melt temperatures, this would lead to increased rates of degradation of the polymer and hydrolysis stabiliser and cause gel formation. Gel formation is much less problematic in the manufacture of other polyester products, such as injection moulded PBT products, in part because of the much greater thickness of those products compared to polyester film.